Modulating radar reflector



vDec. 27, 1966 CHAPMAN, JR 3,295,132

MODULATING RADAR REFLECTOR Filed Feb. 23 1965 4 Sheets-Sheetl ROTATING MEANS Aubrey I.Chapman, Jr.

INVENTOR.

ROTATINGL g MEANS 4 1965 A. l. CHAPMAN, JR 3,295,132

MODULATING RADAR REFLECTOR Filed Feb. 23, 1965 4 Sheets-Sheet 3 QED I7 ROTATING I/5 MEANS Aubrey Chapman,Jr.

INVENTOR.

Dec. 27, 1966 A. l. CHAPMAN, JR 3,295,132

MQDULATING RADAR REFLECTOR Filed Feb. 23. 1965 4 Sheets-Sheet s V Aubrey I. Chapman,Jr.

INVENTOR.

Dec. 27, 1966 A. l. CHAPMAN, JR

MODULATING RADAR REFLECTOR 4 Sheets-Sheet 4 Filed Feb. 23, 1965 Aubrey l. Chapman,Jr.

INVENTOR.

United States Patent 3,295,132 MODULATING RADAR REFLECTOR Aubrey I. Chapman, Jr., Dallas, Tex., assiguor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Feb. 23, 1965, Ser. No. 434,182 g 11 Claims. (Cl. 343-18) This invention relates to a reflector for reflecting radiant energy incident thereupon and more particularly to a microwave reflector for reflecting incident microwave energy thereupon.

The present invention provides a reflector which has reflecting surfaces or strips surrounding a spherical lens which modulates radiant energy incident thereupon. By varying the width and spacing of the reflecting surfaces, the reflector may be coded to provide identification by an aircraft or a vessel, for example. Accordingly, such a reflector may be used to identify an airport or a harbor or a coast line marker. By using a plurality of such reflectors, a landing system for aircraft can also be devised which provides both the ground path and the descent angle for an aircraft.

It is therefore an object of the present invention to provide a reflector which will modulate incident energy thereupon at a predetermined frequency, or frequencies.

Another object of the present invention is to provide a microwave reflector which is of relatively small size and simple in design.

A further object of the invention is to provide a rotating microwave reflector capable of reflecting a plurality of frequencies which can be used to identify an airport, a harbor, coast line marker or other objects, terrestrial or spatial.

A still further object of the invention is to provide a microwave reflector capable of being used a a landing aid in an aircraft landing system.

For a more complete understanding of the present invention and for further objects, features and advantages thereof, reference may now be had to the following description taken in conjunction with the appended claims and accompanying drawings, in which:

FIGURE 1 is a pictorial view of one embodiment of the microwave reflector of the present invention;

FIGURES 2a and 2b are sectional views taken along the section line 22 of FIGURE 1;

FIGURE 3 is a pictorial view of another embodiment of the invention showing a microwave reflector capable of modulating energy at different frequencies;

FIGURE 4 is a pictorial view of yet another embodiment of the invention showing a microwave reflector capable of modulating three different frequencies;

FIGURES 5a and 5b each pictorially represents a microwave reflector, both of which can be used in combination to provide an aircraft landing system;

FIGURE 6 is a pictorial view of yet another embodiment of the invention showing a microwave reflector especially suited for use as a helicopter landing aid.

Referring now to FIGURE 1, there is illustrated a passive microwave reflector 1 constructed according to the teachings of the present invention. This reflector comprises a spherical lens 2 with reflecting surfaces 3 disposed thereon. A spherical lens 2 which may be used in the present invention is, by Way of example, a Luneberg lens, which is a microwave lens of dielectric material, spherical in shape. This type of lens possesses the propcity of focusing an incident plane wave of radiant energy upon a part of its spherical surface to a point on the opposite surface; or, conversely, produces a plane wave from a point source upon its surface.

Disposed upon the spherical lens 2 are reflecting sur* faces 3 which extend substantially from one pole of the 3,295,132 Patented Dec. 27, 1966 lens and terminate substantially at the opposite pole. These reflecting surfaces may be metallic strips which are painted upon or otherwise deposited or attached to the surface of the lens. However, they may also be metallic strips displaced slightly from the lens. Thus at least two modes of construction are possible. Firstly, the reflecting surfaces may be made an integral part of the lens by the placement of metallic paint thereon and therefore both the lens and reflecting surfaces will rotate together as a single unit. Alternatively, the reflecting surfaces may be displaced from the surface of the lens, and accordingly it is possible to rotate the reflecting surfaces only while the spherical lens remains stationary. In order to modulate the incident energy 4 upon the reflecting surfaces, the reflector 1 is rotated by rotating means 5. The focusing and reradiating action of the reflector is shown in FIGURES 2a and 2b and explained below in connection therewith.

FIGURES 2a and 2b show an equatorial cross-section of the reflector 1 of FIGURE 1, indicating how the modulation of the radiant energy incident thereupon occurs. In the operation of the reflector 1, parallel rays of microwave energy 4, for example, from a source, such as an aircraft, ship or the like (not shown) are radiated toward the reflector as shown in FIGURES 2a and 2b. As shown in FIGURE 2a, these parallel rays are refracted by parts of the spherical lens 2 not covered by the reflecting strips 3 so as to converge at a point 6 on the lens. Thus, when the lens is in the position shown in FIGURE 2a, the microwave energy is transmitted therethrough and dispersed into the surrounding medium with substantially no energy reflected back to the source.

FIGURE 2b illustrates the phenomenon when, upon rotating the reflector 1 about an axis perpendicular to the plane of the drawing, the reflecting strips are in a position to present a reflecting strip at point 6. Incoming microwave energy 4 enters the spherical lens 2 between the reflecting surfaces 3 and converges to a point 6 on a reflecting surface 3, from which point the energy is reradiated through the spherical lens and reflected back along the path 4 in parallel rays.

Hence the incident microwave energy will alternately be reflected (FIGURE 2b) and be passed through the lens (FIGURE 2a). Accordingly, modulation of the energy occurs and the frequency of modulation is determined by the number of bands of reflecting surfaces multiplied by the rotational speed of the reflector 1.

As may be seen from FIGURE 1, the reflecting surfaces 3 disposed upon the spherical lens 2 where the incident energy 4 strikes, obscure, on the average, one-half of the effective aperture of the lens. This can be compensated for by a slight increase in lens diameter if so required.

Thus, the reflector 1, operating in the manner described, may be used to modulate radar energy, for example, at a given frequency, said frequency being dependent upon the rotational speed of the reflector and the number of reflecting surfaces associated therewith. Proper detection of the radar returns allows recognition of the reflector. The reflector may be coded by varying the spacings and widths of the reflective surfaces 3 and thus provide a more sophisticated identification of the reflector. This coded recognition can be used to identify a reflector located at an airport, a harbor or a coast line point, each reflector varying in speed of rotation or the size or number of the reflecting strips from every other reflector to provide a distinct modulation of the incident energy for identification on a radar PPI or other appropriate frequency detection means.

FIGURE 3 illustrates another reflector 7 capable of modulating electromagnetic energy, for example, at different frequencies. The reflector 7 comprises a spherical .lens 2 of similar design to the one used in the reflector 1 hereinabove described. In the embodiment of FIGURE 3, the reflecting surfaces 8 extending from one pole of the spherical lens 2 terminate in the equatorial region of the sphere. A second group of reflecting surfaces 9 extending from the opposite pole of said lens also terminates in the equatorial region of the sphere. The number of reflecting surfaces 8 in the upper hemisphere is different from the number of reflecting surfaces 9 in the lower hemisphere, and the surfaces in both hemispheres may be displaced relative to one another, as shown in FIGURE 3. This reflector is rotated by rotating means around an axis through the poles of the reflector.

The operation of reflector 7 is basically the same as the operation of reflector 1, previously described. With reflector '7 spinning about its axis, energy reradiated from each of the different hemispheres will be at a different frequency. That is, incident energy 10 will be reflected and modulated by the lower reflecting strips 9, returning along the path 16', while incident energy 11 will be reflected and modulated by reflecting strips 8, returning along the path 11. Since the frequency of modulation is dependent upon the number of reflecting surfaces (which are different) and the rotational speed of the reflector, the frequency of modulation of the reflected rays of energy 10' will be different from that of reflected rays 11'. Thus a plane is established through the center of the spherical reflector 7 perpendicular to the spin axis. Incident energy entering on one side of the plane is reradiated at one modulation frequency, while incident energy entering upon the other side of the plane is reradiated at a different modulating frequency. incident energy entering parallel to this plane will be modulated with both frequencies.

FIGURE 4 illustrates a microwave reflector 12 capable of reflecting energy incident thereupon at three different frequencies. A spherical microwave lens 2 is surrounded by first reflecting surfaces 13 extending from one pole of said reflector, second reflecting surfaces 14 extending from the opposite pole of said reflector, and third reflecting surfaces 15 surrounding the equatorial region of the reflector. The number of surfaces in said first, second and third reflecting surfaces are unequal in this embodiment. This reflector is also rotated by a rotating means 5 about an axis through the poles of the reflector.

As was described with respect to reflector 7, incident energy 16 is reflected and modulated by reflecting surfaces 14, returning along the path 16', as will incident energy 17 be reflected and modulated by reflecting surfaces 13, returning along the path 17'. Radiant energy 18, perpendicular to the axis of rotation, will be reflected and modulatel by reflecting strips 15, returning along the path 18. Therefore, depending upon the angle of incidence of the incoming energy, any one of three frequencies will be reflected :back to the source.

FIGURES 5a and 5b demonstrate how two of the reflectors 19 and 30 can be used in a landing system for aircraft equipped with forward-looking or weather radar. Aircraft with such radar will have as part of that system means (not shown) for generating and transmitting electromagnetic energy and means (not shown) for receiving and detecting the energy which is reflected. Such generating and receiving means are now standard equipment on many aircraft. FIGURE 5a illustrates a reflector 19 oriented with its axis of rotation perpendicular to the length of the runway 20. With the reflector oriented in this manner, an aircraft 21 may establish a ground path in line with the runway 2G. If the aircraft is too far to the left or right, it will receive different modulation frequencies or radar energy 22 or 23.

FIGURE 5b illustrates how with another similar reflector 3th capable of producing three modulation frequencies differing from those of reflector 19, a glide slope or angle of descent for aircraft Zll can be obtained. In FIGURE 5b, reflector fail is constructed similar to reflector 19, but with the number of reflecting surfaces or revolutions per minute differing from that of reflector 1? to obtain the three different modulation frequencies required.

The axis of rotation 31 of reflector 30 is approximately perpendicular to the ground 32, varying slightly with the vertical, this variation determining the angle of descent of aircraft 21. When the aircraft generates a radar signal along the path 33, it will be reflected and modulated by surfaces 34 along the path 33'. When this particular modulation frequency is received and detected, the radar will determine that the aircraft is too high and will, accordingly, provide information required by the aircraft to lower its descent angle until the radar transmits incident energy along a path 36 and receives a modulation frequency from reflecting surfaces 35 along the path 36. This path defines the correct angle of descent, and by maintaining it, the aircraft will descend at the correct angle. When the aircraft generates a signal along the path 37, which is modulated by reflecting surfaces 38 and reflected back along the path 37, the aircraft will determine it is too low and accordingly will correct its angle of descent.

Thus by using the reflectors 19 and 30 jointly, an aircraft 21 has both ground path and descent angleinformation for an instrument landing without the need for electronic equipment on the ground. The information from the reflectors could be displayed to the pilot on a standard ILS indicator or could be fed to the autopilot.

FIGURE 6 illustrates a reflector 40 particularly suited for helicopter landings. This reflector is similar in construction to reflector 12 except that the upper reflecting surfaces 41 cover more than the hemisphere. Accordingly, reflecting surfaces 42 cover less of the other hemisphere. Reflecting surfaces 43 are disposed in the belt between said reflecting surfaces 41 and 42.

With this arrangement and with the axis of rotation perpendicular to the ground as shown, a cone 44 instead of a plane can be established to provide a glide slope for 360 in azimuth which is particularly useful in helicopter applications.

Assuming that most helicopters make their approach at an angle 0 with the horizontal (see FIGURE 6), a reflector can be designed such that all radar energy entering the reflector on the boundary; 44 of the cone will be at desired descent angle 0 and will be modulated by reflecting surfaces 43 at predetermined modulation frequency. If the radar energy comes in at an angle other than the angle defined by the boundary of the cone 44, such as energy rays 45 and 46, for example, then said rays will be modulated and reflected at different frequencies determined by reflecting surfaces 42 and 41, respectively.

The descent angle 0 of the cone 44 can be varied as required by varying the length that reflecting surfaces 41 and 42 cover the reflector.

Having described the invention in connection with several embodiments thereof, it will be understood that further modifications will suggest themselves to those skilled in the art and it is intended to cover such modifications as fall within the scope of the appended claims. For example, by increasing the number of reflecting surfaces surrounding the lens, more than three frequencies may be modulated and reflected from the reflector of the present invention.

What is claimed is:

1. A reflector for reflecting radiant energy incident thereupon at different frequencies, comprising a spherical lens having opposite poles, first reflecting surfaces in associative relation with said lens, said first reflecting surfaces extending substantially from one pole and terminating substantially a predetermined distance from said pole, second reflecting surfaces in associative relation with said lens extending substantially from the opposite pole and terminating substantially a predetermined distance from said opposite pole, and means for rotating said reflector around an axis through said poles, whereby radiant energy incident upon said first reflecting surfaces is modulated at one frequency and radiant energy incident upon said second reflecting surfaces is modulated at a second he quency,

2. A reflector as defined in claim 1 in which the number of said first reflecting surfaces exceeds the number of said second reflectin surfaces.

3. A reflector as defined in claim 1 in which said first and second reflecting surfaces each covers a hemispherical portion of said lens.

4. A reflector for reflecting radiant energy incident thereupon at dilferent frequencies, comprising a spherical lens having opposite poles, first reflecting surfaces in associative relation with said lens extending substantially from one pole and terminating substantially a predetermined distance from said pole, second reflecting surfaces in associative relation with said lens extending substantially from the opposite pole and terminating substantially a predetermined distance from said opposite pole, and means for-rotating said reflecting surfaces around an axis through said poles to modulate radiant energy incident upon said first reflecting surfaces at one frequency and modulate radiant energy incident upon said second reflecting surfaces at a second frequency.

5. A reflector for reflecting radiant energy incident thereupon at different frequencies, comprising a spherical lens having opposite poles, first reflecting surfaces in associative relation with said lens extending substantially from one \pole and terminating substantially a predetermined distance from said pole, second reflecting surfaces in associative relation with said lens extending substantially from the opposite pole and terminating substantially a predetermined distance from said opposite pole, third reflecting surfaces positioned between said first reflecting surfaces and said second reflecting surfaces, and means for rotating said reflector around an axis through said poles to modulate radiant energy incident upon said reflecting surfaces.

6. A reflector as described in claim 5 wherein said third reflecting surfaces surround the equatorial region of said spherical lens.

7. A reflector as described in claim 6 wherein said first and second reflecting surfaces terminate adjacent to said third reflecting surfaces.

8. A reflector for reflecting radiant energy incident thereupon at different frequencies, comprising a spherical lens having opposite poles, first reflecting surfaces in associative relation with said lens, said first reflecting surfaces extending substantially from one pole and terminating substantially a predetermined distance past the equatorial region of said lens, second reflecting surfaces extending substantially from the opposite pole, third reflecting surfaces positioned between said first reflecting surfaces and second reflecting surfaces, and means for rotatinlg said reflector around an axis through said poles to modulate radiant energy incident upon said reflecting surfaces.

'9. A reflector as described in claim 8 wherein said first and second reflecting surfaces terminate adjacent to said third reflecting surfaces.

10. An aircraft landing system comprising in combination means in an aircraft for generating and transmitting electromagnetic radiation, at least one rotating microwave reflector including a spherical lens and reflecting surfaces in associative relation with said lens, said reflector being arranged to reflect radiation from said first mentioned means in at leasttwo beams having different modulations, and mean-s for receiving and detecting in said aircraft the energy reflected from said at least one microwave reflecting means.

11. An aircraft landing system comprising in combination means in an aircraft for generating and transmitting electromagnetic radiation, a first and second rotating microwave reflector including a spherical lens and reflecting surfaces in associative relation with said lens, each reflector arranged to reflect radiation from said first mentioned means in at least two beams having dif ferent modulations which determine ground path and descent angle of said aircraft, and means for receiving and detecting in said aircraft the energy reflected from said first and second reflectors.

References Cited by the Examiner UNITED STATES PATENTS 2,448,016 '8/1948 Busignies 343'6.5 2,461,005 2/1949 Southworth 343-6.5 2,572,043 10/ 1 McElhannon 343-18 2,580,921 1/1952 Iams 343-48 2,835,891 5/1958 Peeler 343-911 X 2,921,305 l/l960 Cole 343l8 CHESTER L. JUSTUS, Primary Examiner.

G. M. FISHER, Assistant Examiner. 

1. A REFLECTOR FOR REFLECTING RADIANT ENERGY INCIDENT THEREUPON AT DIFFERENT FREQUENCIES, COMPRISING A SPHERICAL LENS HAVING OPPOSITE POLES, FIRST REFLECTING SURFACES IN ASSOCIATIVE RELATION WITH SAID LENS, SAID FIRST REFLECTING SURFACES EXTENDING SUBSTANTIALLY FROM ONE POLE AND TERMINATING SUBSTANTIALLY A PREDETERMINED DISTANCE FROM SAID POLE, SECOND REFLECTING SURFACES IN ASSOCIATIVE RELATION WITH SAID LENS EXTENDING SUBSTANTIALLY FROM TH OPPOSITE POLE AND TERMINATING SUBSTANTIALLY A PREDETERMINED DISTANCE FROM SAID OPPOSITE POLE, AND MEANS FOR ROTATING SAID REFLECTOR AROUND AN AXIS THROUGH SAID POLES, WHEREBY RADIANT ENERGY INCIDENT UPON SAID FIRST REFLECTING SURFACES IS MODULATED AT ONE FREQUENCY AND RADIANT ENERGY INCIDENT UPON SAID SECOND REFLECTING SURFACES IS MODULATED AT A SECOND FREQUENCY. 